1. Technical Field
The present invention relates to a liquid droplet ejection apparatus comprising a liquid droplet ejection head that ejects liquid droplets, a conveyor belt that retains and conveys a recording medium while causing the recording medium to face the liquid droplet ejection head, and a charge roll that causes the recording medium to be electrostatically attracted to the conveyor belt.
2. Related Art
In inkjet printers serving as liquid droplet ejection apparatus, a charge roll presses a paper against a conveyor member such as a conveyor belt or conveyor drum and charges the paper, whereby electrostatic attractive force is created between the paper and the conveyor member and causes the paper to be attracted to the conveyor member. Then, the paper is passed through a region where ink droplets are ejected onto the paper from a recording head, and an image is recorded on the paper. Thus, there is greater uniformity of the distance between the paper and the nozzle surface of the recording head (called “throw distance” hereinafter), the precision of the landing positions of the ink droplets on the paper is improved, and image quality is improved.
In order to increase the uniformity of the throw distance, making the electrostatic attractive force between the paper and the conveyor member stronger is required. And in order to ensure that the uniformity of the throw distance does not drop due to environmental changes, such as changes in temperature and humidity, and to differences in the type of paper, making the electrostatic attractive force between the paper and the conveyor member even stronger is required. In recent years, in order to further improve the precision of the landing positions of the ink droplets on the paper and achieve better image quality, the distance between the paper and the nozzle surface of the recording head has been reduced to 1 to 2 mm.
For this reason, as shown in FIG. 10, the electrostatic attractive force created between a recording head 32 and a conveyor belt 28 becomes strong and the conveyor belt 28 may rise and contact the recording head 32. When the conveyor belt 28 contacts the recording head 32, the conveyor belt 28 becomes soiled with ink, the ink moves from one recording head 32 to another recording head 32 via the conveyor belt 28 such that colors become mixed, and foreign matter adhering to the conveyor belt 28 enters the insides of the recording heads 32 through the nozzles. Particularly in a system that uses transparent ink (reaction liquid) that causes an agglutination reaction when it is mixed with yellow (Y), magenta (M), cyan (C) and black (K) inks (two-liquid system) or a system that causes inks of different colors such as black and yellow to mix and react (inter-ink reaction system) with the purpose of improving image quality, when agglutination of and color changes in the ink occur in the recording heads 32, it is difficult to recover the systems.
As shown in FIG. 11, in a normal state of a conventional inkjet printer, the portion where a charge roll 36 and the conveyor belt 28 contact each other has high resistance so that it becomes difficult for electrical charge to move, and the movement of electrical charge between the charge roll 36 and the conveyor belt 28 results from discharge in the region of a tiny gap between the charge roll 36 and the conveyor belt 28. However, when water droplets or ink droplets adhere to the portion where the charge roll 36 and the conveyor belt 28 contact each other, this portion has low resistance (abnormal state), and the movement of electrical charge occurs at this portion. For this reason, as shown in the graph of FIG. 12, the surface potential of the conveyor belt 28 in an abnormal state rises in comparison to that in the normal state, which affects the rising of the conveyor belt 28 and the uniformity of the throw distance.
An abnormal rise in the surface potential of a charged member can be prevented by ensuring that the charge roll does not contact the charged member. However, when the charge roll is configured such that it does not contact the conveyor belt and does not press the paper against the conveyor belt, sufficient attractive force cannot be applied between the paper and the conveyor belt.